1. Field of the Invention:
This invention, in its preferred form, relates to apparatus for laser machining apparatus and more particularly, to a rigid structure for such laser machining apparatus, wherein the laser source is accurately positioned and rigidly mounted with respect to a work piece to be machined. More particularly, this invention relates to apparatus for welding elements made of a volatile material such as Zircaloy, disposed in a machining chamber in a manner to maintain the purity of an environment substantially non-reactive to the material which the work piece is made, and to ensure that the machining chamber and the work piece therein are readily movable with respect to the laser beam.
2. Description of the Prior Art:
The precision laser machining apparatus of this invention relates generally to the manufacture of nuclear fuel bundle assemblies 10 as shown in FIG. 1 of the drawings. As shown, the nuclear fuel bundle assembly 10 is a self-contained unit comprised of a top nozzle assembly 12 and a bottom nozzle assembly 14, between which is disposed a matrix of nuclear fuel rods 18 arrayed in rows and columns and held in such configuration by a plurality of fuel rod grids 16. Though not shown in FIG. 1, control rods are included at selected positions within the array of nuclear fuel rods 18. The assemblies 12 and 14 and the fuel rod grids 16 provide a skeletal frame to support the fuel rods 18 and the control rods. The nuclear fuel bundle assemblies 10 are loaded into predetermined locations within a nuclear reactor and, therefore, the orientation of the fuel rods 18 with respect to each other is rigorously controlled.
The precision laser machining apparatus of this invention is, in one illustrative embodiment thereof, related to the manufacture of fuel rod grids 16 as shown in FIGS. 2A to 2E. The fuel rod grid 16 is of an approximately square configuration, whose periphery is formed by four outer grid straps 22. Each end of an outer grid strap 22 is welded by a corner seam weld 30 to the end of a perpendicularly disposed outer grid strap. A plurality of inner grid straps 20 is disposed in rows and columns perpendicular to each other, whereby a plurality of cells are formed to receive the control rods and the nuclear fuel rods 18. The inner grid straps 20 disposed along the rows and columns have complementary slots therein at each of the points 24 of intersection for receiving a perpendicularly disposed inner grid strap 20. An intersect weld 32 is formed at each of the points 24 of intersection, whereby a rigid egg crate structure is formed. Further, each of the inner grid straps 20 includes at each end a pair of tabs 26 of a size and configuration to be tightly received in either a top or bottom row of slots 28 formed in the outer grid straps 22, as shown in FIG. 2A. A slot and tab weld 34 is effected along the top and bottom rows formed by the slots 28 within the outer grid straps 22. Further, a plurality of guide sleeves 36 is disposed on the sleeve side surface of the fuel rod grid 16 to receive and guide the control rods disposed therein. A series of notch seam welds 40 securely attaches the guide sleeves 36 to corresponding notches 38 formed within the inner grid straps 20. The precision laser welding apparatus of this invention is particularly adapted to perform a series of controlled welding operations whereby each of the welds 30, 32, 34 and 40 is carried out. The precision laser welding apparatus of this invention not only controls the various parameters of generating the laser in terms of the pulse width, the pulse height of each laser pulse, and the number of pulses to be applied to each weld, but also controls the sequential positioning of the fuel rod grids 16 with respect to the laser beam. It is understood that after each such weld, the fuel rod grid 16 is repositioned and/or the focal point of the laser beam changed to effect the particular type of weld desired.
Referring now to FIGS. 2B and 2C, the plurality of resilient fingers 44 is disposed longitudinally of the inner grid straps 20 in a parallel relationship to each other. A pair of spacing fingers 46 is disposed on either side of a corresponding resilient finger 44 and serves along with the resilient finger 44 to provide a resilient grip of the nuclear fuel rods 18 that are disposed within the cell formed by the intersecting inner grid straps 20. A resilient finger 44a is disposed to the right as seen in FIG. 2C in an opposing relationship to the spacing finger 46a, whereby a nuclear fuel rod 18 is resiliently held therebetween.
The manner of assembling the inner grid straps 20 to each other as well as to the outer grid straps 22 is shown in FIG. 2D. Each of the inner grid straps 20 includes a plurality of complementary slots 52. An upper grid strap 20a has a downwardly projecting slot 52a, whereas a lower grid strap 20b has a plurality of upwardly oriented slots 52b of a configuration and size to be received within a corresponding slot 52a of the inner grid strap 20a. At each end of the inner grid strap 20, there is disposed a pair of the tabs 26 to be disposed within corresponding slots 28 of an outer grid strap 22.
As will be explained in detail later, the inner grid straps 20 are welded to each other by the intersect welds 32 as formed of projection tabs 48 and tab portions 50a and 50b. More specifically, a projection tab 48 is disposed between a corresponding set of tab portions 50a and 50b when the inner grid straps 20a and 20b are assembled together. Upon the application of a laser beam to the tab 50a and tab portions 48 and 50b, an intersect weld 32 is formed that is rigidly strong and free of contamination in accordance with the teachings of this invention. Further, each end of an outer grid strap 22 has a corner tab 54. As shown in FIG. 2D, the outer grid straps 22c and 22b have respectively corner tabs 54b and 54c that overlap each other and are seam welded together to form the corner seam weld 30.
The vanes 42 project, as seen in FIGS. 2C and 2E, from a vane side of the fuel rod grid 16 to enhance the turbulence of the water passing over the nuclear fuel rods 18. Further, as illustrated particularly in FIG. 2C, the guide sleeves 36 are aligned with cells formed by the inner grid straps 20 that are free of either a resilient finger 44 or spacing finger 46, to thereby permit the free movement of the control rod through the cell and through the guide sleeve 36.
Laser machining systems have been adapted for precision work including the cutting of semiconductor assemblies. When lasers are applied to such high precision machining operations, it is important that the relative position between the laser and the work piece, and more particularly between the laser and the work piece holding assembly, be accurately and precisely maintained. Unlike a mechanical tool, which is brought into actual contact with the work piece, a laser is removed from its work piece by a relatively great distance. In such laser machining systems, the work holding means is disposed at a relatively great distance from the laser. In such machines, any relative movement between the work holding means and the laser is amplified by the support structure, causing relatively large movement of the laser beam with respect to the work piece, and thus reducing the accuracy with which the laser beam is focused onto the work piece. Relative vibrations between the laser and the work holding means is serious in proportion to the desired small cross section of the laser beam at the work piece. With accuracies measured in thousandths of an inch or less, it is seen that if there is relative vibration, the focused beam will likewise be vibrated, such vibration being amplified by the structural support system by which the laser and the work piece position means are coupled together. For these reasons, vibration has resulted in problems in obtaining the desired accuracy in laser machining, especially where laser beams are focused to very fine points. Such accurate focusing of the laser beam is advantageously done over a relatively long focal length but increases the difficulty of achieving accurate machining due to vibration and shocks. U.S. Pat. No. 3,803,379 of McKay suggests the use of a rigid construction of a laser optical system mounting bed and a work piece holding frame. In particular, the bed is constructed as a hollow box and interconnected by locator pins forced fit through holes in one member and being threaded in the other member. In addition, the laser mounting bed and the work piece holding frame are mounted on vibration isolating pads to support the entire weight of the bed on a rigid floor.
In the initial development of laser machining systems, lasers were employed for individual, low production machining operations. Typically, single machining operations, e.g. welding, were carried out on a single work piece, one at a time. In order to improve the laser machining rate of the work pieces, the prior art as illustrated by U.S. Pat. Nos. 4,223,201 and 4,223,202 of Peters et al., and U.S. Pat. No. 4,083,629 of Kocher et al., disclose a time sharing of a laser beam emitted from a single laser to be alternately directed along first and second optical paths onto a single work piece. In accordance with the teachings of this invention, the machining rate is further improved by time sharing of a single laser to be directed along first and second optical paths from the laser source to first and second work stations.
In addition, the problems of mounting the laser source with respect to not only one but two work pieces is further complicated by making the work piece of a volatile material such as Zircaloy. It is contemplated that unless the laser machining of such a volatile material is carried out in a substantially pure environment, e.g. an inert gas such as argon, the work piece will be contaminated. In the particular example wherein it is desired to laser weld a Zircaloy work piece, it is contemplated that unless the welding environment is of the highest purity, that any resultant weld will be subject to contamination and that such a weld when subjected to the harsh environment of a nuclear generator will rapidly deteriorate and structurally fail. In one illustrative embodiment of this invention, the work piece as suggested above takes the form of the fuel rod grid 16 for mounting and spacing critically fuel rods within a nuclear generator. If the welds of the fuel rod grid were contaminated, such contamination would lead to the structural failure of the fuel rod grid in such a harsh environment, whereby the flow of water as directed through the nuclear fuel bundle assembly would cause the fuel rods to vibrate against each other with the almost certain result that the fuel rods would rupture and the nuclear material therein be discharged into the circulating water. To prevent this result, the laser welding is carried out in accordance with the teachings of this invention in a welding chamber that maintains the machining environment therein of sufficient purity to prevent the undesired contamination, but also to permit the machining chamber and the work piece disposed therein to be controllably moved with respect to the laser beam. In this manner, the laser source and the work piece positioning means may be disposed under the automated control of the computer to thereby further increase the machining rate of the work pieces.